1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus for reading an image while dividing the image into a plurality of areas.
2. Related Background Art
In an image reading apparatus used in a facsimile apparatus or a digital copying machine, a photoelectric conversion element such as a CCD is used. In this case, since sensitivity with respect to an input of an element is varied even among single elements depending on their positions due to the characteristics of the optical system and the conversion element, a correct image signal cannot be obtained. For this reason, a shading circuit is normally arranged to perform electrical correction so as to obtain apparently uniform sensitivity. More specifically, reference white and black data are read by the element, and a mathematical operation is performed so that these data become uniform data among pixels. Sampling of reference data for calculating a corrected value is performed by reading a standard white plate provided at a home position of an original table prior to reading of an original image. Thereafter, an actual original is read.
In order to read an original image having a size larger than an original size capable of being read by the photoelectric conversion element, the original image is divided into a plurality of stripe areas, and images are read in units of areas, as disclosed in U.S. patent application Ser. No. 798,672, and the like.
However, when a large-size original is input by scanning it in a plurality of lines, the following phenomenon is caused as the number of scanning lines is increased.
A mechanical displacement of an optical system due to flexure of an original table is increased as it is separated from the home position where shading data is sampled. Along with a lapse of time until a far area is scanned, a change over time in temperature of an input system is also increased.
Therefore, input sensitivity is changed from an initial state by the above-mentioned phenomenon, and correct shading is disturbed. More specifically, since the input sensitivity of a reading range tends to be slanted in one direction as compared to an initial state, a discontinuous offset appears at a boundary of the scanned areas, and a scanned image is different from an actual image. In this manner, since a boundary line having a discontinuous density appears on an image, degradation of image equality is conspicuous, resulting in a so-called shading error.
In order to avoid this, the following means are proposed.
(i) A reading area of an input element is widened, and an image is input without being divided.
(ii) Before the shading error occurs, shading data is re-sampled.
Of these means, the means (i) is a drastic countermeasure. However, it is very difficult to manufacture an element having a size capable of reading an A1-size original, resulting in high cost. Since the processing circuit becomes large in size accordingly, the means (i) is not a practical means.
On the other hand, the means (ii) does not have large difficulties in technique and cost. However, since sampling of shading data takes much time, a reading speed of the reading apparatus is decreased.